Emergency locking safety belt retractors are in widespread use throughout the world in vehicle occupant restraint belt systems. Many such retractors have a mechanism for automatically locking the reel against rotation in the unwinding direction in response to acceleration of the vehicle that includes an inertia responsive actuating device which detects acceleration of the vehicle and actuates a locking mechanism associated with the belt take-up reel.
There are basically three types of inertia responsive actuating devices. The first is the pendulum type in which a mass is suspended from a support, commonly a cap that lies above the support and carries a dependent post to which the mass is affixed. Compared to the other two common types of inertia responsive actuating devices, the pendulum type has the disadvantage of requiring an extra assembly operation, that of affixing the mass to the post after the post has been inserted through the hole in the support. The additional cost of that step, although small on a case-by-case basis, adds up to a considerable overall cost over the production of many tens of thousands of devices.
The second general category of devices is the ball type. In its simplest form, the ball type device includes a pawl or other actuator having a horizontal follower surface which rests on top of the ball and is pushed up by wedging action of the ball as the ball rolls up an inclined surface on a support. The wedging action of the ball when it accelerates due to inertial force relative to the support is opposed by the mass of the pawl acting down at a contact point on the pawl and friction at the contact points between the ball, on the one hand, and the support and the pawl, on the other hand. Ordinarily, the frictional force acting at the contact point between the ball and the support is substantially larger than frictional force at the contact point between the ball and the pawl. Therefore, the ball rolls up the inclined surface of the support unless the direction of the inertial force includes an upward component sufficient to reduce or eliminate the frictional force. The effect of friction at the contact point between the ball and the pawl can be shown to be a function of the angle (at any given position) between a line connecting the two contact points and a line perpendicular to the surface of the ball at the contact point between the ball and the pawl, an angle that can be termed the "pressure angle."
In the simple case under consideration, the pressure angle, and therefore the friction, is initially relatively small and increases as the ball rolls up the inclined surface in any direction. As the ball moves from the rest position, the acceleration of the pawl varies appreciably, depending upon the direction of ball motion relative to the pawl pivot for any given acceleration of the ball. Similarly, the amount of displacement of the tip of the pawl in a given time varies substantially with the direction of ball movement. Accordingly, the responsiveness of the simple form of ball type device is highly variable and constitutes an important disadvantage.
Some of the disadvantages of the ball type inertia responsive actuating device can be overcome by providing a cam surface on the ball, such as a downwardly facing conical surface on the ball or an annular follower flange. Such arrangements can be designed to reduce somewhat the variations in response, that is, the directional variation. On the other hand, the initial acceleration of the pawl for a given acceleration of the ball is relatively great, inasmuch as the contact point between the ball and the pawl is located at a point on the surface of the ball where the tangent plane is substantially inclined to the horizontal. As the ball rolls from rest to maximum operating positions, the acceleration of the pawl decreases as a function of displacement. In addition, the pressure angle, and therefore the effect of friction, is at a maximum in the rest, non-actuating position and diminishes at a diminishing rate as a function of displacement. Inasmuch as acceleration of the pawl and the effect of friction are greatest in the rest position, the force required to start the ball rolling is high relative to the force required to keep it rolling. Coupled with the fact that static friction is substantially greater than kinetic friction, this type of actuating device is subject to considerable variation in the time when motion begins following an acceleration of the ball due to inertia in response to acceleration of the support. When the actuator output element is a pawl, the device is still highly directional insofar as the amount of displacement of the pawl for a given displacement of the pawl. Such variation can be eliminated by providing a vertically displaceable slider instead of a pawl, but ordinarily such a slider works against a pawl and such an arrangement requires more parts and other problems and disadvantages.
The third type of inertia responsive actuating device, the standing weight type, comprises a mass of any suitable shape having a stem extending down from the bottom that rests in a socket in a support that keeps the mass from moving appreciably in any horizontal direction but allows the mass to tip over from a standing, upright position. A camming surface, for example, an upwardly facing conical surface on top of the mass, works against a follower projection on a pawl or slider. Such a standing weight device has the advantage of being substantially uniformly responsive to acceleration in any direction, inasmuch as the follower projection on the pawl remains generally centered vertically over the axis of the socket in the base of the support. Such devices can also be designed to provide predictable rates of response and favorable friction characteristics (low starting friction, in particular). They are of relatively simple, inexpensive construction. They have, on the other hand, an important disadvantage, namely, a high hysteresis. The hysteresis problem is perhaps best understood by imagining that the device is slowly tilted from horizontal just to the point when the weight tips over. For example, let it be assumed that the vertical axis tilts 15.degree. before the mass tips over. Having reached that position, the mass then tips through the design angle of tilting within the casing, say 5.degree.. Before the weight will return to the upright position on the base, the casing must be tilted back down by the same 5.degree., and the 5.degree. can be termed the hysteresis. As a practical matter, the relatively high hysteresis results in the retractor being subject to remaining in locked condition when the vehicle stops on a downward incline. In the example, the retractor is prone to staying locked when the vehicle stops on a downward incline of more than about 10.degree..